1. Field of the Invention
This invention relates to an image forming apparatus such as a laser beam printer, and more particularly to an image forming apparatus such as a laser beam printer which is capable of processing data at a relatively high speed in accordance with a data transmission speed provided from a host system to the laser beam printer through an interface operative in a high speed data transmission mode such as ECP (extended capabilities port) defined by IEEE (Institute of Electrical and Electronics Engineers).
2. Discussion of the Background
A new parallel port interface operative for data transmission between a host system and an image forming apparatus such as a laser beam printer has recently been standardized by IEEE (Institute of Electrical and Electronics Engineers). This interface is called IEEE-1284 and includes a new bidirectional communication mode generally referred to as ECP (extended capabilities port). A maximum of 2 Mbytes per second is a data transmission speed featured by ECP, which is faster than a popular interface known as Centronics connecting an image forming apparatus such as a laser beam printer to a personal computer and transmitting data at 100 to 150 Kbytes per second as a maximum speed. ECP, in addition, features a bidirectional communication capability although it has a relatively simple specification similar to that of the background Centronics, a uni-directional interface, and is thus called bi-directional Centronics to differentiate from the background uni-directional Centronics.
Data transmitted from a host system to an image forming apparatus generally includes two types of data. One type of data is defined as print data which includes image data, image form related data such as a carriage return code, and the like. These print data are needed to be processed together in an originally received sequence. The other type of data is defined as control data for requesting various actions, such as, a cancel data command for canceling designated data, a status request command for requesting printer status information, and so forth. These control data can also be classified by levels of urgency requesting immediate actions. As an example, the above-mentioned cancel data command may be classified as a highly urgent command because if the action of the image forming apparatus in response to this command is delayed, then the canceled data may possibly be printed and the command consequently fails. A paper tray selection, as another example for the command classification, may be classified because the paper tray selection is needed only when the image forming apparatus starts printing after completing data processing. When such data including print data and control data is transmitted from a host system to a background image forming apparatus through ECP mentioned above, the background image forming apparatus receives the data and then transmits the received data into a RAM (random access memory) thereof. For the data transmission to the RAM, the background image forming apparatus generally adopts DMA (direct memory access) since DMA is a high speed data transmission and host independent operation. Because of this host-independent operation, the load of the CPU of the background image forming apparatus is not increased by DMA. In the event of the data transmission, if data storage in the RAM is made without distinguishing commands from print data, the background image forming apparatus can not transmit the print data from the RAM to a print unit before separating the print data and control data. The background image forming apparatus thus causes problems in executing DMA and actions requested by control data.
There has been proposed a solution to obviate the above-mentioned problems. An image forming apparatus having this solution can determine whether received data from a host system represents print data or control data by arranging a CPU thereof to check a so-called Host Ack signal transmitted together with the data from a host system for indicating whether the data under transmission is print data or control data. On the other hand, when utilizing this solution the CPU increases its load with increasing data transmission speed through a high speed data transmission interface such as ECP.
Furthermore, since the load to the CPU of the background image forming apparatus is variant, the CPU does not always have enough power for processing immediate actions requested by the control data. In such a situation, an event may subsequently be caused in which the CPU only processes the control data in a temporary manner and leaves a remaining part of the actions requested by the control data to be processed later. A resultant delay in processing the control data may pose a serious problem, particularly in a case that immediate actions are requested by the control data.
Because of these problems in processing data, a great delay has resulted in the development of an image forming apparatus such as a laser beam printer including a data transmission interface compatible to the new high speed data transmission interface such as ECP of IEEE-1284 defined by IEEE. Subsequently, an image forming apparatus that obviates the above-mentioned problems, or that is capable of processing data including print data and control data at a relatively high speed in accordance with a data transmission speed from a host system using ECP has not yet been developed.